1. Field of the Invention
The present invention relates to a scanning probe microscope for obtaining observed images of samples by using physical phenomena (e.g., a tunnel current and an interatomic force) observed when a probe is moved closer to the samples, and a method of observing samples by using this microscope.
2. Related Background Art
Recently, a scanning tunneling microscope (STM) capable of directly observing the electron structure of a surface atom of a conductor has been developed and is expected to be put to use in a wide variety of applications for the reasons explained below.
(1) The STM can observe real space images of samples with a high resolving power regardless of whether the samples are single-crystal or amorphous.
(2) The STM has an advantage that it can observe samples with a low electric power without damaging them by an electric current.
(3) The STM can be operated even in the atmosphere and can be used for various materials.
For example, G. Binning et al., Helvetica Physica Acta, 55, 726, 1982 reports that a molecular image of an organic molecule adsorbed in the surface of a conductor was observed by using the STM.
The STM makes use of a tunnel current that flows between a metal probe (probe electrode) and the surface of a conductive substance when the probe is moved closer to a distance of about 1 nm from the surface of the conductive substance with a voltage applied between the probe and the substance. The magnitude of this tunnel current depends on the distance between the probe and the surface of the conductive substance and is very sensitive to the change in distance. For this reason, while controlling the distance between the probe and the surface of the conductive substance so that the tunnel current is maintained constant, the probe is scanned two-dimensionally above and relative to the surface of the conductive substance, thereby measuring the real space surface structure (surface undulations) of the conductive substance. At the same time, it is possible to read various pieces of information concerning the entire electron cloud of surface atoms. Note that the resolving power in the direction of the surface of the conductive substance in this case is about 1 .ANG..
Meanwhile, an atomic force microscope (AFM) taking advantage of the technology of the STM has been developed, and this also makes it possible to obtain the surface structures and the like of samples as does the STM (G. Binning et al., Phys. Rev. Lett., 56, 930, 1985). The AFM obtains a real space image of the surface of a sample with a high surface resolving power by using an interatomic force sensitive to the distance between a probe and the surface of the sample. The AFM can perform measurements on the order of atoms even for insulating samples and therefore its further development is expected.
Not only the STM and the AFM described above but microscopes, which obtain distance information between a probe and the surface of a sample by using a physical quantity produced by an interaction between the probe and the surface of the sample and sensitive to the distance between them, or microscopes, which obtain a real space image of the surface structure or the surface condition of a sample with a high resolving power by maintaining the above-mentioned physical quantity constant by feedback control, are generally called scanning probe microscopes. These scanning probe microscopes are being developed as the technology derived from the STM.
FIG. 1 is a view for explaining the locus of a probe in obtaining the surface structure of a sample by using a scanning probe microscope.
Referring to FIG. 1, the distance between a probe 1 and the surface of a sample 10 is maintained constant at any instant by maintaining a physical quantity (e.g., a tunnel current or an interatomic force) sensitive to the distance between the probe 1 and the surface of the sample 10 constant by feedback control. Consequently, when the probe 1 is scanned in the direction indicated by an arrow shown in FIG. 1 above the surface of the sample 10 having a small projection 11.sub.1 and a small recess 11.sub.2, a locus L of the probe 1 traces the surface structure of the sample 10 as indicated by a broken line in FIG. 1. Therefore, an image representing the surface structure of the sample 10 can be obtained from a signal for scanning the probe 1 and a signal indicating the amount of feedback control for the physical quantity.
The above-mentioned conventional scanning probe microscope, however, has the following two significant problems.
The first problem is that since the scanning of the probe 1 is effected while feedback control is performed for the distance between the probe 1 and the surface of the sample 10, the scan rate of the probe 1 must be increased in order to shorten a time required to obtain the overall surface structure of the sample 10. When, however, the scan rate of the probe 1 is raised too high, the surface structure of the sample 10 cannot be obtained accurately for the reason explained below if a large projection or the like exists on the surface of the sample 10.
If a large projection 12 that the feedback control cannot follow exists between the projection 11.sub.1 and the recess 11.sub.2 on the surface of the sample 10 as shown in FIG. 2, the locus L of the probe 1 becomes the one indicated by a broken line in FIG. 2 when the scan rate of the probe 1 is raised too high. As a result, even when image processing is performed on the basis of the signal indicating the amount of feedback control for the physical quantity, no indication of the recess 11.sub.2 present on the right side of the projection 12 in FIG. 2 appears in the obtained image.
When a recess or a step of a size that the feedback control cannot trace exists on the surface of the sample 10, the surface structure of the sample 10 cannot be obtained correctly for the same reason as described above. In addition, the same problem arises in obtaining a real space image of the surface condition of a sample by using the conventional scanning probe microscope.
As described above, the conventional scanning probe microscope obtains a real space image of the surface structure or the surface condition of a sample by scanning a probe. Therefore, a time required to obtain the real space image of the surface structure or the surface condition of the sample is determined by the scan rate of the probe. In addition, since scan of the probe is performed while the distance between the probe and the surface of the sample is maintained constant by feedback control, the scan rate of the probe is limited according to the relationship between the frequency band of a feedback control circuit, which is limited by the mechanical resonance frequency of a probe scanning mechanism, and the spatial frequency that the probe must follow, which is determined by undulations on the surface of the sample. This introduces the following problems.
(1) To obtain a real space image of the surface structure or the surface condition of a sample at a high speed, it is required that no projection, recess, nor step, that the feedback control cannot trace, be present on the surface of the sample.
(2) When the scan rate of the probe is decreased to ensure the followability of the feedback control, a distortion may be produced in the resulting real space image of the surface structure or the surface condition of a sample because the secondary positional relationship is not expressed correctly owing to a thermal drift or the like.
The second significant problem is as follows. In scanning the probe 1 while performing feedback control for the distance between the probe 1 and the surface of the sample 10, the scan of the probe 1 is normally performed by moving the probe 1 or the sample 10 in the scan direction by using a piezoelectric element. In this case, however, a certain phenomenon (drift) occurs in which the relative positional relationship between the probe 1 and the surface of the sample 10 is gradually changed by the creep phenomenon of the piezoelectric element or the change in the ambient temperature. As an example, if a drift in which the relative positional relationship between the probe 1 and the surface of the sample 10 gradually widens takes place, the locus L of the probe 1 declines to the right, as shown in FIG. 3. This makes it impossible to accurately obtain a real space image of the surface structure of the sample 10.
When the above-mentioned drift is found in the conventional scanning probe microscope, therefore, a measurement of a real space image of the surface structure or the surface condition of the sample 10 is started after the resulting image is stabilized. As a consequence, it is not possible in practice to make the best use of one of the advantages of the scanning probe microscope over a scanning electron microscope, a short measurement time.